Analysis of such a sample as DNA and protein is growing in importance in recently years, and this requires a DNA sequencer to provide a higher speed and greater throughput. One of the examples is found in a capillary array electrophoresis apparatus having multiple capillaries arranged in a plane surface. The capillary array electrophoresis apparatus basically comprises a capillary array, an excitation optical system consisting of a laser light source and the like, and a light receiving optical system for detecting fluorescent light. The capillary array consists of capillaries arranged in a plane surface. Laser light is applied to the capillary filled with the sample (fluorescent sample) labeled with fluorophore in the direction parallel to the capillary arranged surface, and laser light is condensed by a lens-like function of the capillary, whereby fluorescent samples in all capillaries are exposed to laser light. The fluorescent sample emits fluorescent light. The light receiving optical system detects the fluorescent light emitted from the fluorescent sample that emits fluorescent light almost normal to the direction of the laser light coming from the laser, thereby measuring the sample. However, part of the fluorescent light of the fluorescent sample emitted due to application of laser light is reflected by the capillary surface. In the capillary array electrophoresis apparatus disclosed in Japanese Application Patent Laid-Open Publication No. Hei 09-096623, this reflected light is detected by the light receiving optical system to become one of the causes for increase of the background light, resulting in deterioration of the detection accuracy. To solve this problem, the following configuration is disclosed in the U.S. Pat. No. 5,790,727: Laser light is applied to the capillary filled with fluorescent sample in the direction parallel to the surface where multiple capillaries are arranged. In a capillary array electrophoresis apparatus where a light receiver receives the fluorescent light of the fluorescent sample emitted in the direction normal to laser light, a light extractor consisting of an optical fiber is mounted on the aforementioned capillary in the direction where fluorescent light is detected, in such a way that the light reflected from the capillary surface does not enter the light extractor consisting of an optical fiber. Then the fluorescent light traveling inside the optical fiber is received by the fluorescent light detector, thereby reducing the background light.
According to the prior art disclosed in the aforementioned U.S. Pat. No. 5,790,727, however, an optical fiber is mounted on the fluorescent light detector in order to ensure that the fluorescent light reflected from the capillary surface is not detected by the light receiving optical system, thereby reducing the background light and improving the detection accuracy. While the capillary array is a consumable product, the fluorescent light detector consisting of an optical fiber is a fixture of the apparatus. Accordingly, every time the capillary array is replaced, it is necessary to align the capillary center axis of the capillary array fluorescent light detector and the center axis of the optical fiber constituting the optical extractor. This means a time-consuming work for a user. Further, detection accuracy will deteriorate if misalignment occurs between the center axis of the capillary and that of the optical fiber.